Ion implantation provides a technique for the introduction of controlled amounts of dopants into semiconductor materials in order to modify their electronic properties. A review of ion implantation technology and description of the equipment and processes involved may be found in "Crystalline Semiconducting Materials and Devices" Plenum Press 1986, Chapter 15, Ion Implantation and Annealing, in "Semiconductor Devices Physics and Technology" S.M. Sze Ed., Wiley 1985, Chapter 10, Diffusion and Ion Implantation, and in Encyclopedia of Chemical Technology, Kirk-Othmer, Ion Implantation pp 706-719. The process involves implantation by means of a high-energy ion beam. Typically, an ion source, such as for example, phosphine, is ionized in an ion implantation apparatus by electron impact, emitted from a hot tungsten source. The ions produced in a high vacuum region are analyzed and mass separated by a magnetic field. The desired ions, such as for example phosphorus, are then accelerated by an electric field to high energies. The resulting ion beam is passed through scanners and allowed to strike the target substrate. Accurate control of ion implantation is possible because (a) the average penetration depth of implanted ions depends on their energies and (b) the total number of ions depends on the beam current. In view of the latter relationship it is important that the beam contains a high percentage of the desired ion species. Thus, for efficient ion implantation it is desirable to have a high total beam current and a high percentage of the desired ion in the beam current.